If a person wants to make a fraud he can open a new account in person. But we have to wait for our rank to increase in order to be able to join signature campaigns.
Only if people trusted newbie accounts! It's hard to farm an account just for signature campaigns as one can easily make out if the account bought.However the case is not the same if a trade is involved with the bought account.People tend to fall for the account ranks or the trust feedbacks.